The present invention relates to the field of semiconductor devices and their manufacture. More specifically, in one embodiment the invention provides both bipolar and complementary metal-oxide (CMOS) semiconductor devices on a single substrate and a process for their fabrication.
Bipolar and CMOS devices and their fabrication have been well known for many years. Recently, the advantages of both types of devices have been beneficially incorporated into circuits using both types of devices on a single substrate. Circuits which incorporate both bipolar and CMOS devices have come to be known as "BiCMOS." BiCMOS devices offer the advantages of the high packing density and low power consumption of CMOS devices, as well as the high speed of bipolar devices. One BiCMOS device and process for fabrication thereof is described in U.S. Pat. No. 4,764,480 (Vora), assigned to the assignee of the present invention.
While meeting with some success, BiCMOS devices continue to have certain limitations. For example, electrical contacts in BiCMOS devices have caused limitations in their use. One limitation is the current drive capability of CMOS devices which is typically limited by source/drain sheet resistance. Extrinsic base resistance in bipolar transistors is in some instances an important factor in AC performance of bipolar structures. Still further, collector-substrate junction capacitance degrades the speed of bipolar circuits. High resistivity of a polysilicon-silicon substrate contact can limit the performance of especially small devices (e.g., sub-micron). Further, source and drain contacts in previous devices were spaced an undesirable distance from the gate of MOS transistors. Also, ground tap resistance may be a limiting factor in removal of injected current from the substrate in vertical fuse devices.
From the above it is seen that an improved BiCMOS device and method of fabrication thereof is desired not only to provide devices with improved performance and reduced size, but also to provide devices which can be fabricated more quickly and economically.